The ferrate ion, FeO.sub.4.sup.-2, in which iron exhibits a valence of six is a powerful and useful oxidizer. A major problem, however, in the production of ferrates using aqueous process is that ferrates are unstable. Specifically, alkali metal and alkaline earth metal ferrates, i.e. salts having the anion FeO.sub.4.sup.-2 are relatively stable when dry, but when dissolved in water, they decompose rapidly.
Ferrates have many known uses such as those described in U.S. Pat. No. 4,405,573 by Deininger et al., issued Sep. 20, 1983. U.S. Pat. No. 4,405,573 is incorporated herein by reference in its entirety. For example, as described in U.S. Pat. No. 2,758,090 by Mills et al., issued on Aug. 7, 1956, ferrate has been employed to bleach vegetable fibers, to effect organic reactions and to oxidize sulfurous acid, nitrites, ferrocyanides and other inorganic material. The ferrate(VI) ion has been used to decolorize caustic solutions as disclosed in U.S. Pat. No. 2,536,703 by Schreyer, issued on Jan. 2, 1951; and to oxidize carbohydrates as disclosed in U.S. Pat. No. 3,632,802 by BeMiller et al. issued Jan. 4, 1972.
Ferrates have also been studied for their uses in wastewater treatment. For example, Murmann et al., "Experiments Utilizing FeO.sub.4.sup.-2 for Purifying Water," Water Research, vol. 8, pp. 79-83 (1974) showed the use of ferrate in removing some toxic trace metals from wastewater and Waite, "Feasibility of Wastewater Treatment with Ferrate," Proceedings of the American Society of Civil Engineers, Vol. 105, No. EE6, December 1979 showed the use of ferrate use in removal of suspended solids, phosphate, ammonia, and the disinfection of bacteria in wastewater.
In addition to these uses, it has recently been discovered that wastewater solutions containing radioactive transuranic elements and compounds can be cleaned by precipitating transuranics therefrom using ferrate. This process is described in U.S. patent application Ser. No. 07/349,285, entitled "Method of Treating Wastewater", now U.S. Pat. No. 4,983,306 issued Jan. 8, 1991 the disclosure of which is incorporated herein by reference in its entirety.
One problem preventing the widespread use of such processes is that ferrates are difficult to produce, particularly in commercial quantities. In the late 1940's, Schreyer developed a laboratory method for the production of potassium ferrate(VI). In this method, a sodium hypohalite or halogen gas (e.g. NaOCl or Cl.sub.2) is reacted with a ferric salt in an aqueous NaOH solution to produce Na.sub.2 FeO.sub.4. The Na.sub.2 FeO.sub.4 is then converted to potassium ferrate(VI) by the addition of KOH. Schreyer et al., "Potassium Ferrate(VI)", Inorganic Synthesis, Vol. IV, pages 164-169 (March 1951).
Another laboratory method which employs a hypohalite/ferric salt reaction technique involves the direct reaction of potassium hypochlorite, potassium hypobromite or a halogen gas with Fe(OH).sub.3 in the presence of an alkali metal hydroxide to form and precipitate K.sub.2 FeO.sub.4. U.S. Pat. No. 2,455,696 by Mosesman issued Dec. 7, 1948; and Audette et al., "Potassium, Rubidium, Cesium and Barium Ferrate(VI): Preparation, Infrared Spectra and Magnetic Susceptibilities," Inorganic Chemistry, Vol. 11, No. 8, pages 1904-1908 (1972).
There are a number of problems with the above methods for the production of ferrates which are based on the reaction of an alkali metal hypohalite with an iron-containing compound. First of all, ferrate(VI) is unstable in aqueous solution and rapidly degrades to produce ferric hydroxide, particularly in the presence of even small amounts of metallic or organic impurities. Second, large amounts of waste material are produced leading to economic inefficiencies and an acceleration in the decomposition of the ferrate ions. Third, the hypohalite (e.g. NaOCl) is unstable. Fourth, excessive heat is generated during the reaction which can rapidly degrade the ferrate and the hypohalite. Finally, excessive amounts of potassium chloride salt and the like are produced as byproducts, particularly in the production of potassium ferrate.
Other methods for producing ferrates include the electrolysis of iron-containing materials in a electrolytic cell containing KOH or the like, or by fusing iron or ferric oxide with potassium nitrate in the presence of KOH. See Mellor, A Comprehensive Treatise on Inorganic and Theoretical Chemistry, pages 929-937, Longmans, Green & Co., London (1952) and U.S. Pat. No. 4,435,257 by Deininger et al., issued Mar. 6, 1984.
In U.S. Pat. No. 4,435,257 by Deininger, an electrolytic process for producing sodium ferrate in a membrane-type electrolysis cell is described. Specifically, an electrolytic cell is charged with an aqueous solution of sodium hydroxide, a sodium halide salt and ferric ions (Fe.sup.3+) in the anolyte chamber and sodium hydroxide solution in the catholyte chamber. While it is not certain how sodium ferrate is produced in this process, a theory is disclosed that the iron anode or iron salt is converted in the electrolysis process or by reaction with OH.sup.- ions, into ferric oxyhydroxide [e.g. Fe.sub.x O.sub.y .multidot.nH.sub.2 O where n is greater than 1], then electrochemically converted in the presence of the halide ion to ferrate ion.
The above-described methods of ferrate production by electrolysis and the fusing of an iron source with potassium nitrate also have significant disadvantages. The direct electrolysis method is commercially impractical for continuous operation because of the passivation of the iron anode by the formation of a ferric oxide film which causes an increase in voltage and a decrease in ferrate production. In addition, the yields produced by this type of process are small. With respect to fusing iron or ferric oxide with potassium nitrate to produce ferrates, this method requires high temperatures and also produces a small yield.
Other processes for producing alkali metal ferrates are described in U.S. Pat. No. 4,385,045 by Thompson, issued May 24, 1983 and U.S. Pat. No. 4,545,974 by Thompson, issued Oct. 8, 1985. The first process involves subjecting a particulate reactant mixture of elemental iron and an alkali metal peroxide to high temperatures in the substantial absence of free oxygen. The second method involves reacting an alkali metal nitrate or alkaline earth metal nitrite with hematite (Fe.sub.2 O.sub.3), magnetite (Fe.sub.3 O.sub.4), or an iron compound which self-reacts via thermal decomposition at a temperature less than about 1100.degree. C. to form Fe.sub.2 O.sub.3, followed by subjecting the reactants to high temperatures in the range of about 780.degree. C. to about 1100.degree. C. for a specified period of time. These processes have the disadvantages of requiring energy-consuming high temperatures and producing low yields of product. In addition, the resulting product of these processes is a mixture of iron(IV) ferrates and iron(VI) ferrates, rather than a substantially pure iron(VI) ferrate product.
A process for producing potassium ferrate is also described in U.S. Pat. No. 4,405,573 by Deininger et al., issued Sep. 20, 1983. The process of this patent involves the production of potassium ferrate by the reaction of a substantially pure ferric salt, substantially pure Cl.sub.2 and aqueous KOH containing less than 10 parts per million by weight of total harmful metallic and organic impurities through the reaction of intermediates KOCl and Fe(OH).sub.3 in the presence of a stabilizing proportion of an alkali metal silicate.
The process described in this patent has several disadvantages. First, the addition of ferric salt directly to the hypochlorite reactor generates excessive heat which degrades both hypochlorite and ferrate, resulting in reduced yields and increased byproduct salt wastes. Second, the process of this patent includes washing with organic solvents which are difficult to dispose of and remain in the ferrate product in trace amounts. These trace amounts result in an unstable ferrate product with a relatively short shelf-life. Typically, such an unstable ferrate product is unsuitable for use in wastewater treatment applications. Third, it is difficult to use recycled liquids in the process of this patent as the proper water balance cannot be easily maintained, and evaporative removal of water is nearly impossible.
None of the above described methods provide for the production of ferrate in an economic manner on a commercial scale (i.e. 0.25 tons to 100 tons per day). Therefore, it would be advantageous to provide a commercially feasible production method for ferrates which results in high yields of high purity ferrates in an economic and efficient manner. As used herein, the term "high purity ferrates" means a ferrate product which is substantially Fe(VI), rather than other iron forms such as Fe(III). High purity does not relate to the amount of ferrate which is produced by the process of this invention as compared to other acceptable byproducts such as potassium chloride. The term "commercially feasible production" means production of ferrates in daily quantities of at least approximately one hundred pounds per day with minimal byproduct formation and waste disposal problems.